1. Technical Field of the Invention
The present invention relates to a laser scanning apparatus in which a laser beam from a light source is scanned by a rotational scanning member such as a polygon mirror.
2. Brief Description of the Prior Art
In a laser scanning apparatus, a polygon mirror which is suitable for high speed scanning is being widely used as a scanning member, wherein all the polygonal surfaces are utilized as mirrors and are capable of successively scanning the number of times corresponding to the number of polygonal surfaces during a single rotation. The balance during rotation is maintained stably with the least generation of vibration and noise, and its high efficiency is most advantageously utilized in high speed scanning.
Heretofore, for instance, U.S. Pat. No. 3,750,189 proposes a method of correcting any irregularities in the planar surfaces of a polygon mirror, tilting of the surface of each mirror surface and the like by utilizing a lens system. However, it is unable to sufficiently rectify such problems as the tilting of surface and the like with such a structure. On the other hand, a highly accurate manufacturing process is required for manufacture of the polygon mirror, since the surface of the polygon mirror affects the quality of the image to be formed. In spite of such requirements, polygon mirrors provided with as many surfaces as 6 to 12 facets have conventionally been utilized with an extraordinary expensive cost which occupies most of the cost in an optical system of laser beam printers and the like.
However, utilization of a mirror with a few surfaces may also be taken into consideration. For instance, when the following conditions are set,
______________________________________ System Speed 30 mm/s Writing Density 300 dot/inch Number of surfaces of 6 polygon mirror ______________________________________
the number of revolutions necessary for the polygon mirror will be 3,543 r.p.m. However, this speed does not create a problem, since a drive motor used for a similar type polygon mirror is capable of sufficiently rotating the mirror for about 10,000 r.p.m., and the motor is available at the same cost. By increasing the number of revolutions of the polygon mirror within a range of the limit, the number of surfaces of the polygon mirror can be decreased, thereby lowering the total cost of an apparatus. Accordingly, polygon mirrors having 2 or 3 surfaces may also be utilized hypothetically.
However, with a decrease in the number of surfaces, the polygon mirror gradually becomes angular in shape. For instance, it becomes a triangle with 3 surfaces and a flat board with two surfaces, and it becomes difficult to cope with the noise of air generated at the time of high speed rotation. Moreover, the vibration increases when the mirror is moved, thereby affecting the quality of image and durability of the apparatus.